Electrostatic printing is a well known printing process in which an electrostatic latent image is made to attract electrostatic marking particles, so called toner. The toner can be of the dry type or of the liquid type. Electrostatic printing is particularly applicable when only a relatively small number of prints are required, or when the subject matter is frequently changed, or when part of the subject matter needs to be sequentially changed.
Dry powder toners have many disadvantages when used in such a process. The main objection are related to the dusting problem; dust, or fine, or small particles of toner are prone to escape from the developer, and these deposit onto any surface both within and outside the printing device, causing mechanical failures within the device and environmental problems outside the device. This problem becomes severe when such printing devices are run at high speed. Other disadvantages include, cost of the general maintenance of the press and cost of the dry powder toner.
Liquid electrostatic printing also has a number of objectionable problems, especially when these devices are required to operate at high speed. The main problem is in regard to the solvent carry-out. The term solvent carryout relates to the quantity of solvent or carrier which is trapped within the paper and mechanically removed from the toner applicator. Such solvent subsequently evaporates, giving rise to atmospheric pollution and also adding significantly to production costs. A further disadvantage of liquid toning is the tendency for deposition of colouring matter in non-image or background areas which results in a general discolouration of the copy, normally referred to as background fog.
Various methods have been proposed to reduce solvent carry-out and background fog. Of these methods the most significant appears to be that disclosed in U.S. Pat. No. 4,268,597 of Klavan et al, in which a thin layer of liquid dispersed toner is formed at a development zone, and the surface on which a physical material image is to be formed is disposed close to but out of contact with the toner dispersion at the development zone. The electrostatic field associated with the latent image on the surface causes a change of shape of the surface of the layer of liquid dispersed toner in the form of pseudopods which are conjointly in the shape of portion of the latent image. The pseudopods contact the latent image bearing surface in the image areas only causing toner deposition in the image areas only.
Klavan et al disclose many mechanisms for formation of a thin layer of liquid dispersed toner at the development zone. However no attempt is made to pre-dispose the toner particles to develop the latent image on the surface to be toned, the developing member being partially immersed in a body of liquid dispersed toner and subsequently rotated against a doctor blade or roller or series of rollers to produce a dispersed toner layer of the desired thickness at the development zone.